In known in-vehicle radars, detection is performed over a detection range in front of a vehicle, and the position and the velocity of a target such as another vehicle existing in the detection range are measured. In FM-CW radars, a transmit signal is transmitted which alternately has an up modulation period in which frequency gradually increases and a down modulation period in which the frequency gradually decreases. If a reflected signal from a target is received, the position and the velocity of the target with respect to the position and the velocity of the vehicle are determined on the basis of a frequency spectrum of a beat signal due to the difference in frequency between the transmitted signal and the received signal.
In the FM-CW radars described above, the relative velocity (the “Doppler velocity”) is measured on the basis of the frequency shift of the beat signal appearing in the frequency spectrum due to the Doppler effect, and the position of the target are determined every predetermined measurement interval and the moving velocity (the “differential velocity”) of the target is determined from a change in the position. Each of these two types of velocity information has advantages and disadvantages, and use of only one of these two types of velocity information can cause a problem in the determination of the velocity.
For example, Patent Document 1 discloses a technique to determine whether Doppler velocity data is reliable enough, on the basis of differential velocity data. In this technique, if it is determined that the Doppler velocity data has an unreliable value, the Doppler velocity data is corrected using the differential velocity data. More specifically, when the Doppler velocity is greatly different from the differential velocity, the differential velocity is employed as the relative velocity of a target, while when the difference between the Doppler velocity and the differential velocity is small, the Doppler velocity is employed as the relative velocity of the target. Alternatively, when the difference between the Doppler velocity and the differential velocity is great, a value employed for an immediately previous relative velocity is employed as a current value of the relative velocity, while when the difference between the Doppler velocity and the differential velocity is small, the Doppler velocity is employed as the relative velocity of the target.
Patent Document 2 discloses a technique to obtain a high-accuracy relative velocity by correcting a velocity determined from a Doppler shift frequency on the basis of a differential velocity.
Patent Document 1: Japanese Examined Patent Application Publication No. 2695086
Patent Document 2: Japanese Unexamined Patent Application Publication No. 07-146358
If an abrupt change occurs in a reflection point on a reflection object such as a vehicle running ahead, the differential velocity becomes worse in accuracy than the Doppler velocity. In the method disclosed in Patent Document 1, a great error occurs in such a situation, because the differential velocity is employed when the difference between the Doppler velocity and the differential velocity is great. Besides, when there is a great difference between the Doppler velocity and the differential velocity, use of the previous relative velocity in determining the differential velocity causes the resultant differential velocity to have a great error compared with the Doppler velocity, if the target has great acceleration.
In the radar disclosed in Patent Document 2, when a target moves in a region close to a continuously extending reflection object such as a guard rail, a large error occurs in the Doppler velocity (and a large error can occur also in the differential velocity), and thus it is difficult to obtain accurate velocity information.